Calcium enhances Acanthamoeba polyphaga binding to extracellular matrix proteins.

PURPOSE To characterize better the ameba-host interactions that may be involved with the pathogenesis of Acanthamoeba keratitis, the role of calcium (Ca2+) on the binding of Acanthamoeba polyphaga to extracellular matrix proteins was examined in vitro. METHODS The binding of a metabolically labeled A. polyphaga (CDC:0187:1) isolate from a case of human keratitis to collagen type IV, laminin, and fibronectin was assessed through a range of calcium concentrations in the external fluid. Binding to collagen IV was studied in detail, with and without other divalent cations and calcium channel modulators. RESULTS Calcium increased binding in a dose-dependent manner, with significant effects at 0.1 to 1.0 microM and near-maximal effects at 1 to 100 microM, depending upon the matrix protein. Magnesium alone had no effect on ameba binding to collagen IV but suppressed the action of calcium. Strontium enhanced ameba binding, with maximal effect at 100 microM. The calcium channel antagonists nifedipine and diltiazem-HCl and a calcium channel activator, Bay-K8644, had no effect on the action of calcium. However, the inorganic calcium antagonists, lanthanum and cobalt, suppressed the effect of calcium. CONCLUSION Low concentrations of calcium enhance the adhesion of A. polyphaga to extracellular matrix proteins. It remains uncertain whether calcium acts intracellularly or at the cell surface.